This invention relates to the reciprocation of a fluid pressure drive unit, and more particularly to a switching valve for effecting automatic reciprocation of a fluid pressure piston-cylinder unit.
Automatic reciprocation of an hydraulic or other fluid pressure piston-cylinder unit heretofore has been effected by having the piston rod mechanically operate a valve to switch the fluid back and forth. U.S. Pat. Nos. 4,143,760 and 4,619,819 describe this method. In some instances it is undesirable to have external mechanism to do this shifting. Accordingly, valves have been built to sense the working pressure of the cylinder, and when a preset pressure level slightly higher than the highest working pressure is reached, switching takes place to reverse the motion. U.S. Pat. No. 2,711,717 describes this method. This arrangement works well when the load on the working cylinder is relatively constant and there is only a slight additional pressure rise needed to cause shifting of the valve.
However, the above valve system is undesirable when an uneven load is placed on the cylinder. This is because the valve must be set to switch at a pressure slightly higher than the highest pressure required. When the cylinder has little load on it and needs almost no pressure to operate, but still the valve requires the highest pressure setting in order to switch, there is a major pressure rise at the time of shifting.
To illustrate, when an internal combustion engine is used to drive a fluid pressure pump and there is a maximum load on the cylinder, the engine is working quite hard. Accordingly, the additional slight pressure rise required for shifting the valve above this working pressure will bring about shifting without being noticed. However, if the load diminishes and the engine is working at idle, the requirement of having to build a high pressure to shift the valve may stall the engine before it can throttle up for a full load, or at the least will cause surging of the engine in an undesirable manner.